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Removal of ammonium and phosphates from wastewater


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  • 1. Water Research 37 (2003) 1601–1607 Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing by-product !J.M. Chimenosa, A.I. Fernandeza, G. Villalbaa, M. Segarraa, A. Urruticoecheab, B. Artazab, F. Espiella,* a Department of Chemical Engineering and Metallurgy, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain b ! Asistencia Tecnologica Medioambiental, S.A., Epele Bailara, 29, 20120 Hernani, Spain Received 24 April 2001; received in revised form 29 March 2002; accepted 30 September 2002Abstract The wastewater produced by the cochineal extract process to obtain the carminic acid colouring pigment (carmin redE120) has high concentrations of phosphates and ammonium. It is known that both ions can be precipitated withmagnesium in the form of struvite, MgNH4PO4, or ammonium magnesium phosphate (MAP) compounds. In thisstudy, the use of an alternative MgO-containing by-product is investigated. The optimal pH, reaction time and solid/liquid ratio have been studied. It has been found that the low-grade MgO needed is greater than the stoichiometric valuefor the full removal of ammonium and phosphate as MAP compounds. Although the low-grade MgO (LG-MgO) reactsslower than pure MgO, it has considerable economic advantages. A batch process has been proposed for the removal ofammonium and phosphates from wastewater obtained in cochineal extracts processing, previously to biologicaltreatment to diminish the COD.r 2002 Elsevier Science Ltd. All rights reserved.Keywords: Struvite; Map; Cochineal extracts; Ammonium removal; Phosphate removal1. Introduction ammonium and phosphates as well as high concentra- tion of soluble chemical oxygen demand (COD). Nowa- The cochineal insect, dactylopius coccus, is the raw days, the wastewater is being treated in a subcontractedmaterial in the production of carmine lake, a natural red water treatment plant where the legal requirements fordyestuff (E120) obtained from the carminic acid. It is its discharge are met. However, the company, considersused principally as a colouring agent in cosmetics, that having its own treatment plant will make the wholebeverages and products with low pH. The colour hue process more environmentally friendly and will haveranges from orange to red as a result of the different important economic savings. In this wastewater treat-modes of cochineal extraction. ment plant the ammonium and the phosphates must be CHR Hansen is among the world’s largest producers removed in a physico-chemical step prior to a conven-and suppliers of natural carmine lake, mainly produced tional biological its factory located in Spain. The wastewater obtained The removal of phosphorus has been largely studied,at the end of the process has high contents of and at present, there are two effective and reliable methods established, chemical precipitation and biolo- *Corresponding author. Tel.: +34-3-4021316; fax: +34-3- gical removal [1]. In most chemical treatments, the4021291. phosphorus can be removed from sewage by precipitat- E-mail address: (F. Espiell). ing via a metal salt, i.e. iron, aluminium and mainly0043-1354/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 5 2 6 - 2
  • 2. 1602 J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607calcium salts [2–5]. The phosphorus removal through In this study, experiments to remove phosphates andbiological treatment has been developed during the last ammonium from cochineal insect processing wastewatertwenty years and is now beginning to compete with the are performed using an LG-MgO. The aim of this studymore conventional physico-chemical approach of pre- has been to determine the optimum parameters atcipitation with metal salts, mainly in municipal waste- laboratory scale, needed for a further design of awater and animal manure treatments [6,7]. Chemical and physico-chemical pilot plant installed prior to conven-biological removal methods both allow phosphorus to tional biological treatment plant. A reaction mechanismbe recycled as a sustainable product for use as raw involving LG-MgO to form MAP compounds ismaterial in industrial or agricultural applications [8,9]. proposed in accordance with the obtained results. Ammonium is a common parameter in industrialsewage, and a biological treatment plant can onlyremove N–NH3 concentration of down to 200 mg/L. 2. Methods and materialsHigher concentrations of ammonium must be dimin-ished prior to a biological treatment. This may be This study was carried out with the wastewater fromaccomplished through a stripping step of ammonia, the cochineal insects processing, developed by CHRgenerated at a pH higher than 9.2, with an airflow which Hansen, to produce natural carmine (E120).must be rinsed later to remove ammonia contents. The factory, located in Navarra (Spain), processes Nevertheless, it is well known that ammonium and 800–1000 kg of cochineal insects per day to obtain 135–phosphates can be precipitated together with magnesium 165 kg of carminic acid as the precursor of the differentin the form of struvite, MgNH4PO4, or ammonium carmine lakes commercialised. The extraction processmagnesium phosphate (MAP) compounds. [10–13]. from the scale of cochineal insects to obtain carmineStruvite or MAP precipitation has been mainly used lake uses ammonium hydroxide as extracting agent andprior the biological treatment of animal manures or phosphoric acid as acidifying agent. At the end of themunicipal wastewater. Moreover, phosphorous and process, 35,000 L of wastewater per day are producedammonium recovered as MAP compounds may find with a high content of phosphates and ammonium asan application in the fertiliser sector as a slow release well as the high COD. A sample of 50 L of wastewaterfertiliser. obtained from the optimum and representative condi- There are many advantages in using MgO as raw tions of the process was used to perform laboratorymaterial, namely, magnesium oxide has minimal envir- trials. The chemical bulk analysis of an aliquot fromonmental impact, has a low solubility, and is essential initial wastewater is shown in Table 1.for plant, animal, and human growth; also, it has a high LG-MgO used as source of magnesium is producedalkalinity, more than other alkalis, which helps to and sold by Magnesitas Navarras, S.A. located inneutralise acids and precipitate metals requiring lessMg(OH)2 to neutralise the same amount of acid;Mg(OH)2 is a weak base and its dissolution is not Table 1exothermic, reaching a maximum pH of 10 which is in Composition of initial wastewater from the cochineal insectsorder with the Clean Water Act basic limits; the sludge processing to produce natural carmine (CHR Hansen) and low-formed by the reaction is conducive to crystal growth grade magnesium oxide (Magnesitas Navarras)and is not light, fragile, or gelatinous like that formed by Wastewater Low-grade MgOother alkalis [14]. While the use of struvite for the removal and recovery pH 2.1 MgO (%) 70.0of phosphates and ammonium is technically feasible for CaO (%) 9.7the treatment of high strength wastewater, it is not N–NH3 (mg/L) 2320 SiO2 (%) 4.2 Al2O3 (%) 2.7adopted economically since the high cost of magnesium P–PO3À (mg/L) 4 3490 Fe2O3 (%) 0.6compounds [15], i.e. magnesium hydroxide chemical SO3 (%) 4.9reactive is eight to ten times more expensive than similar COD (mg O2/L) 10205 d100 (mm) 100quality of calcium hydroxide. However, to obtain d50 (mm) 8struvite, it is possible to use other sources of magnesium Ca2+ (mg/L) 42 d10 (mm) 3that are more economically feasible. In this way, low- Bulk density (g/cm3) 3.0grade magnesium oxide (LG-MgO) may be used for the SO2À (mg/L) 4 3458 LOI (11001C) (%)b 8.9removal of phosphates and ammonium as MAP BET (m2/g) 8.3compounds. Nevertheless, if the struvite formed is used TSS (mg/L)a 160later as slow release fertiliser, the other compounds a TSS: Total suspended solids.contained in LG-MgO must be natural, insoluble or b LOI: Loss of ignition.stable substances in the working media, i.e. having very dx : Accumulated fraction lower than particle size.low concentrations of heavy metals. BET: Specific surface area measured by single point BET.
  • 3. J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607 1603Navarra (Spain). The initial price of this low-grade 3. Results and discussionproduct (approximately 100 per ton) is close to calciumhydroxide price commonly used in the wastewater The initial wastewater has an ammonium/phosphatetreatment plants. It comes from the calcination in rotary molar ratio of 1.5:1 Considering that the molar ratiokiln at 11001C of natural magnesite. The product is NH+:Mg2+:PO3À to form struvite is 1:1:1, the stoichio- 4 4collected as cyclone dusts in the fabric filters from the air metric amount of magnesium oxide necessary to removepollution control system. The bulk composition and all phosphorus contained in wastewater is 4.6 g/L ofother physical parameters are shown in Table 1. The pure MgO or 6.6 g/L of LG-MgO, according with thecontent of calcium oxide is due to the presence of small composition described in Table 1.amounts of dolomite—MgCa(CO3)2—in natural mag- Pure MgO reacted two to three times faster than thenesite, and may contribute to diminish phosphorus as low-grade MgO for pH range lower than 7, as ainsoluble calcium phosphate. The high loss of ignition consequence of the lower particle size and the greater(LOI) value obtained means that the product still BET (m2/g). Nevertheless, in the pH range greater thancontains unburned magnesite and dolomite. The pre- 7, both sources of MgO have similar reactivity to formsence of iron and aluminium may also contribute to struvite. As a result of that, the needed time to obtaindecrease phosphorus concentration forming the insolu- the theoretical optimal pH [12] is lower (approximately 3ble iron/aluminium phosphates. The presence of silica, times) when using pure MgO.from natural origin, does not interfere with the physico- Regarding the decantation rate, pure MgO showedchemical wastewater treatment and remains inert in the worse results than LG-MgO. The sedimentation rate forprecipitated compounds. Finally, it is possible to pure MgO was 15–20 times lower than LG-MgO as aestablish a relation between the reactivity of the LG- function of solid/liquid ratio studied. These differencesMgO and its specific surface area (BET) [16]. In this in the settling velocity may be explained mainly by thecase, the low BET value obtained (8.3 m2/g) means that physical characteristic of both MgO particles. While thekinetics of precipitation of MAP compounds will be mean particle size (d50) for LG-MgO is lower than 8 mm,slower than using pure MgO with a BET value of see Table 1, for the pure MgO is lower than 1 mm.115 m2/g, which is a consequence of the mean particle Using LG-MgO, the effects of S/L ratio and time onsize determined (d50) lower than 1 mm and the high the concentration of nitrogen and phosphorous and pHporosity of the particles. is depicted in Figs. 1–3. Figs. 1 and 2 show respectively The experiment trials were performed using a the phosphate and ammonium concentration in waste-flocculation tester that consists of six 400 mL beakers water versus reaction time as a function of LG-MgOthat are agitated simultaneously at 200 rpm. All experi- added. In both experimental trials the initial amount ofments were carried out at a room temperature of 251C. LG-MgO added ranged from 12 to 34 g/L. That meansDifferent solid/liquid (S/L) ratios (12, 16, 20, 24, 30 and from 1.8 to 5.1 times the stoichiometric MgO needed.34 g/L) and ten reaction times (0.5, 1, 2, 4, 6, 8, 12, 16, 20 For all reaction times studied, it can be observed inand 24 h) were studied. The resulting suspensions Fig. 1 that the phosphate concentration decreases withwere filtered through 45 mm membrane filters and the the increase of the S/L ratio. Moreover, all phosphoruspH was determined from the clear filtrates. The resulting has been removed, excluding the batch experimentclear solution was acidified with concentrated HNO3 performed with 12 g/L, for a reaction time of 24 h.and was used for the analysis of phosphorus and Nevertheless, for S/L ratios greater than 20 g/L theammonium. The phosphorus as phosphate was analysed phosphorus concentration plunges in short reactionby Inductive Coupled Plasma -Atomic Emission Spec- times. For example, phosphorus concentrations undertrometry (ICP-AES) and ammonium by injection flow 50 mg/L were obtained in less than 6 h by adding LG-analysis (IFA). MgO amounts over 20 g/L. An evaluation of decantation rate was performed Regarding the nitrogen removal (stated as ammo-using volumetric laboratory equipment to determine the nium) similar results were obtained while increasing theLG-MgO at different S/L ratio. The progression of solid S/L ratio. For a reaction time of 4 h, the nitrogen (N–sedimentation in a batch process was followed using a NH3) concentration slightly decreases for high LG-MgO2 L graduated cylinder. The results have been compared slurries, or remains steady for LG-MgO slurries lowerto the results obtained using commercial pure MgO than 24 g/L. Nitrogen concentrations under 200 mg/Lobtained by electrofused process. only were obtained with a S/L ratio over 30 g/L and The precipitates obtained were examined by more than 16 h, or 24 g/L and reaction time of 24 hX-ray diffraction (XRD) and scanning electron micro- respectively.scopy (SEM) with energy dispersive spectrometer In Fig. 3 the pH values as a function of LG-MgO(EDS) analyser to determine the different compound added and reaction time are depicted. It can be observedformed and help to elucidate a possible reaction that pH values greater than 8 cannot be achieved with S/mechanism. L ratios lower than 20 g/L. The lowest values for the
  • 4. 1604 J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607 3500 10 12 g/L 12 g/l Phosphorus concentration (mg/L) 3000 16 g/l 16 g/L 9 20 g/l 20 g/L 2500 24 g/l 24 g/L 30 g/l 30 g/L 8 34 g/l 2000 34 g/L pH 1500 7 1000 500 6 0 0 5 10 15 20 25 5 0 5 10 15 20 25 30 Reaction time (h) Reaction time (h)Fig. 1. Variation of the phosphorus (P–PO3À) concentration in 4the natural carmine lake process wastewater as a function of Fig. 3. pH recorder in the natural carmine lake processlow-grade MgO slurries versus reaction time. wastewater treatment as a function of low-grade MgO slurries versus reaction time. 2500 12 g/L 3500 16 g/L Experimetnal values Nitrogen concentration (mg/L) 20 g/L 2000 3000 Theoretical values Phosphorus concentration (mg/L) 24 g/L 30 g/L 34 g/L 2500 1500 2000 1000 1500 1000 500 500 0 0 5 10 15 20 25 0 4 5 6 7 8 9 10 Reaction time (h) pHFig. 2. Variation of the nitrogen (N–NH3) concentration in the Fig. 4. Comparison of phosphorus concentration experimentalnatural carmine lake process wastewater as a function of low- values and phosphorus concentration from struvite solubilitygrade MgO slurries versus reaction time. product versus pH.solubility of MAP compounds were reported in the pH phosphate compounds, as iron or calcium, has not beenrange of 8–10 [11,12,17]. Nevertheless, pH greater than considered. The experimental values analysed at pH9.2 may be attributed to equilibrium solubility of lower than 6.2 indicate that MAP precipitated com-Mg(OH)2 (pKsp=11.1). That means that an excess of pounds exceed the solubility product and the solutionLG-MgO has been added. remains oversaturated, while all phosphorus concentra- The variation of phosphorus (P–PO3À) concentration 4 tions obtained at pH greater than 6.2 lie under thein the wastewater as a function of pH is shown in Fig. 4. calculated solubilities. Iron, aluminium, calcium andIn the same figure, calculated concentrations from carbonate ions contained in LG-MgO, and the calciumequilibrium solubility of struvite have been depicted, contained in the wastewater resulting from the produc-using the solubility product (pKsp=12.6) cited in the tion of the natural dyestuff, as well as the lowliterature [18,19], as well as other parasite reactions as magnesium activity, may contribute to phosphatesthe formation of acid species (NH3, H3PO4, H2POÀ, 4 removal thus varying the equilibrium conditions of theHPO2À and PO3À) and the formation of magnesium 4 4 struvite. Under these conditions, experimental values forphosphate. However, to obtain this theoretical curves pKsp of struvite should be determined. Nevertheless, itthe presence of other ions capable to forme insoluble seems that a struvite solubility control is achieved after
  • 5. J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607 1605 2000 a: Struvite MgNH4 PO4 .6H2O Experimental values a b: Periclase MgO 1800 c: Magnesite MgCO3 ; Theoretical values d: Dolomite MgCa(CO3)2 Nitrogen concentration (mg/L) 1600 e: Quartz SiO2 a b 1400 a Intensity (a.u) 1200 a a 1000 c d a 800 a a b c 600 e a c a a a a a ba d d aa a e b a 400 200 0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 4 5 6 7 8 9 10 2θ pH Fig. 6. X-ray diffractogram of a sludge obtained in theFig. 5. Comparison of nitrogen concentration experimental wastewater treatment with low-grade MgO (24 g/L and 20 h).values and nitrogen concentration from struvite solubilityproduct versus pH.these impurities have reacted forming insoluble phos-phates. Fig. 5 shows the wastewater nitrogen (N–NH3)concentration, obtained from all experimental trialsperformed, as a function of pH. It is always over thetheoretical struvite solubility curve. This fact agrees withthe initial wastewater ammonium/phosphate ratio.Nevertheless, the removed ammonium/phosphate ratio,calculated from concentrations analysed at pH valuesfrom 5.5 to 9, is in accordance with the theoretical molarratio 1:1. So, the removal of ammonium is only due tostruvite-MAP compounds formation, which needs aminimum phosphate concentration to precipitate. Fi- Fig. 7. Scanning electron micrograph of a sludge particlenally, at a pH greater than 9.2 the nitrogen concentra- obtained in the wastewater treatment with low-grade MgO:tion slumps. This fact may be explained by the ammonia inside an MgO periclase zone (a) and struvite growth on thegas formation (pKa=9.2), which is stripped due the low-grade MgO particle surface (b).vigorous reactor agitation. The sludge obtained at the pH considered as optimum(pH 8) was used for the analysis of crystalline mineralphases. X-ray diffractogram of a sample obtained by showed in Fig. 7 that there are two different morphol-adding 24 g/L of LG-MgO to wastewater resulting from ogies, corresponding to inside (a) and outside (b) of thethe production of carmin red dye is shown in Fig. 6. particle. EDS microanalysis from part (a) indicates thanPeaks of struvite were identified as the main phase, as magnesium is the main element contained. Furthermore,well as MgO periclase. Magnesite, dolomite and quartz small peaks corresponding to calcium, silicon, iron andhave been also identified as minor phases present in the aluminium, as well as oxygen, sulphur and carbon havesludge. The identity of other phases present in small also been identified. Nevertheless phosphorus andquantities was very difficult to establish, because the nitrogen have not been identified. These results corro-pattern is characterised by a large number of small borate that the inside of the particle had not reacted withoverlapping peaks. The presence of magnesium as MgO the aqueous medium and remain unchanged. On thepericlase as well as other non-reacted mineral phases other hand, EDS microanalysis from part (b) reveals thefrom initial LG-MgO, suggests that the initial particles important presence of phosphorus and nitrogen togetherhave not totally dissolved and struvite may be formed on with magnesium. Furthermore, small peaks correspond-the particle surface. This is in accordance with the ing to iron and calcium, both from the LG-MgO, haveabsence of brucite Mg (OH) 2. also been identified in the outside of the particle. That This fact has been corroborated by SEM/EDS means that struvite crystals growth on the LG-MgOmicroanalysis performed on microtomed thin-sections particle surface may be blocking the phosphate andof sludge particles. It can be observed in the micrograph ammonium diffusion.
  • 6. 1606 J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607 According to these results, a reaction mechanism is and phosphate from wastewater resulting from theproposed to illustrate the removal of phosphates and production of carmin red dye from cochineal.ammonium using LG-MgO as source of magnesium. At The pH measurement may be used as a controlthe beginning, the Nernst boundary layer interface parameter in a physico-chemical plant to establish theinvolving solid particle is formed and hydrolysis of end of the reaction and the amount of LG-MgO slurryMgO surface takes place. The solubility equilibrium of added. The use of the optimum conditions allows valuesmagnesium hydroxide is achieved and hydroxyl and lower than 35 mg/L of phosphorus (P–PO3À) and 4magnesium concentration increase in the interface. 230 mg/L of nitrogen (N–NH3) in the final water,Next, the diffusion of hydroxyl and magnesium ions to removing the 99 and 90 percent of initial concentrations,the bulk solution takes place. Generally, the rate of these respectively. These minimum ammonium and phosphatesteps is faster when pure MgO is used. However, when concentrations may be removed in a biological plant,using LG-MgO less reactive, these reactions may which is necessary for the removal of COD.become the rate controlling steps. Because of low activity of the LG-MgO used in this Simultaneously, phosphoric acid and ammonium work, the formation of MAP compounds takes place ondiffusion occurs from the bulk solution to the interface. LG-MgO particle surface. This MAP layer blocks theAccording to experimental results obtained, the diffu- diffusion of magnesium from inside of the particle to thesion rate of phosphoric acid and ammonium to the boundary layer stopping the growth of struvite. As ainterface is faster than the diffusion rate of magnesium result of this, it is necessary to add 3.5–4 times moreand hydroxyls to bulk solution. Consequently, the MgO than is stoichiometrically needed, and extra LG-struvite formation takes place in the interface instead MgO remains in the product obtained. Consequently,of the bulk solution. about 45 g/L of dry sludge are obtained. However, In this proposed mechanism only the formation of because of the composition of LG-MgO and the naturalstruvite has been taken into account. However other source of the red dyestuff, both without harmfulmagnesium and phosphate compounds such as formulas substances, the sludge may be used as slow-releasenewberyte and bobierrite are also formed [20,21], and fertiliser.calcium and iron phosphates as well as gypsum may be On the other hand, due to the particle size and specificformed too. gravity of LG-MgO, the decantation rate is greater than The formation and growth of the MAP compounds that obtained by using pure MgO and less volume offorms a layer on the surface particle that does not allow sludge is obtained. This dense sludge can be quickly andthe diffusion of magnesium and hydroxyls thereby easily dewatered.stopping the reaction. As a result of these findings, a batch physico-chemical plant will be installed at CHR Hansen, S.A. (Spain) to treat 35,000 L of wastewater per day. Low-grade MgO used in this experimental work, may4. Conclusions be also used to treat another sort of wastewater containing phosphates and ammonium, i.e. animal It is possible to remove phosphate and ammonium manures or municipal wastewater.from wastewater by precipitation of MAP compoundsusing low-grade MgO as source of magnesium. Due tothe price of different grades of MgO, the use of low-grade instead of pure MgO diminishes the costs of Acknowledgementswastewater treatment in a physico-chemical plant. According to the results above described, 24 g/L of The authors would like to thank Magnesitas NavarrasLG-MgO and a reaction time of 5 h is required to S.A. for its cooperation in financing and supporting theremove all the phosphate as MAP precipitated com- work and CHR Hansen S.A. who kindly providedpounds. In this period of time the pH reaches values logistical support and samples. The authors alsoaround 8.5–9, the value considered as optimum. There is acknowledge to Scientific and Technical Services of thea remainder of ammonium that does not precipitate University of Barcelona for its analytical assistance.after this point; to remove it is necessary increase the pHabove 9.2 so the ammonium is eliminated as ammoniagas. This increase of pH takes place after 15 h due to the Referencespoor reactivity of the low grade MgO. Nevertheless,reaction time is not a critical parameter in this waste- [1] Morse GK, Lester JN, Perry R. The economic andwater treatment, mainly due to the volume of waste- environmental impact of phosphorus removal from waste-water daily produced. Thus, a reaction time of 20 h is the water in the european community. London: Selpertime considered as optimum to remove the ammonium Publications, 1993.
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